Electrode structure for controlling electron flow with high transmission efficiency

ABSTRACT

Electrode structure for controlling flow of electrons from a cathode to an anode, characterized by the absence of electrodes in the flow path whereby electron flow is substantially unimpeded by physical structure and proceeds with high transmission efficiency. The structure is such as to establish an electron accelerating field and associated flow path between cathode and anode which bulges outwardly about an interposed control electrode and is thus not physically intercepted by same. By virtue of the high efficiency of electron transmission, the electrode structure may be advantageously incorporated in an Xray tube to provide X-ray pulses of increased intensity. In addition, the transmission characteristic in conjunction with the capability of the bulging field to enable the use of relatively small control voltages to govern electron flow render the electrode structure particularly well suited to the attainment of a high amplification factor in a high power amplifier tube for voltage regulation or switching applications.

United States Patent Gaines et a1.

[451 May 23, 1972 Mancebo; Frederick D. Cook, all of Livermore, Calif.

[73] Assignee:

The United States of America as represented by the United States Atomic Energy Commission [22] Filed: Dec. 9, 1970 [21] Appl. No.: 96,514

[52] U.S.Cl ..313/55, 313/30, 313/308, 313/313, 313/356 [51] Int. Cl. ..H0lj 35/04 Primary ExaminerJohn Kominski Assistant Examiner-Darwin R. Hostetter Attorney-Roland A. Anderson [57] ABSTRACT Electrode structure for controlling flow of electrons from a cathode to an anode, characterized by the absence of electrodes in the flow path whereby electron flow is substantially unimpeded by physical structure and proceeds with high transmission efficiency. The structure is such as to establish an electron accelerating field and associated flow path between cathode and anode which bulges outwardly about an interposed control electrode and is thus not physically intercepted by same. By virtue of the high efficiency of electron transmission, the electrode structure may be advantageously incorporated in an X-ray tube to provide X-ray pulses of increased intensity. In addition, the transmission characteristic in conjunction with the capability of the bulging field to enable the use of relatively small control voltages to govern electron flow render the electrode structure particularly well suited to the attainment of a high amplification factor in a high power amplifier tube for voltage regulation or switching applications.

8 Claims, 4 Drawing Figures PATENTEDMAY23 I972 3,665 236 sum 1 OF 3 *6 6+ 20 21 13 HIGH I 1K VOLTAGE i BIAS SUPPLY l 12 1 25,4 16 ,19

l :El: 0 0 5 7 CONTROL 23 S IGNAL SOURCE Fig.2

INVENTORS. Jerry L. Gaines Paul J. Ebert LIoyd Mancebo Frederick D. Cook BY ATTORNEY.

PATENTEDMAYZMQYZ 3,665,236

sum 2 0F 5 INVENTORS Jerry L. Gaines Paul J. Ebert Lloyd Mancebo Frederick D. Cook ATTORNEY.

PATENTEUMAY 23 m2 sum 3 UF 3 INVENTORS.

BY /W 4 ATTORNEY.

ELECTRODE STRUCTURE FOR CONTROLLING ELECTRON FLOW WITH HIGH TRANSMISSION EFFICIENCY BACKGROUND OF THE INVENTION The invention disclosed herein was made in the course of, or under Contract U-7405-ENG48 with the United States Atomic Energy Commission.

Electron tubes for a variety of diversified applications, such as the amplification or switching of electrical energy, generation of X-rays, and the like, typically include an electron emissive cathode, an anode spaced from the cathode, and at least one control electrode interposed between the cathode and anode, all of which electrodes are disposed within a vacuum envelope. The anode is generally maintained at a positive potential with respect to the cathode to thereby establish an electrostatic field effective to accelerate electrons from the cathode towards the anode. The amount of electrons reaching the anode is determined by a control signal on the control electrode which may vary between negative and positive values with respect to the cathode potential. When the control signal is negative, the accelerating field is opposed and electrons are repelled from the control electrode back towards the cathode in accordance with the magnitude of the negative signal. When the control signal is zero or positive, the accelerating field is unaffected or aided and the electrons are free to flow to the anode. Thus, in the foregoing manner the electron flow to the anode is governed by the signal impressed on the control electrode.

In such conventional electron tubes, the efficiency of electron transmission from the cathode to anode is substantially limited due to the control electrode structure being physically disposed in the electron flow path. In this regard, the control electrode has been heretofore usually provided as a gridded structure of limited transparency which thus collects a significant portion of the electrons flowing to the anode, even when the control signal is such as to establish full flow of electrons. Although some improvement is derived by utilizing a caged control electrode structure in place of a gridded one, conductor wires of the cage are still physically disposed in the electron flow path. These wires hence intercept and collect some of the electrons and prevent them from reaching the anode, thereby decreasing the transmission efficiency.

In addition to the limited electron transmission efficiency obtainable with conventional electrode structure of electron tubes, it is to be noted that such structure is further disadvantageous from the standpoint of the relatively large swing of control signal potential required to control a given electron current flow between maximum and cut-off. Such potential swing in conjunction with the limited transmission efficiency are limiting upon the amplification factor of an electron tube employing conventional electrode structure.

SUMMARY OF THE INVENTION The general object of the present invention is to provide improved electrode structure for electron tubes wherein the control electrode is not physically disposed in the electron flow path and such path bulges coaxially about the control electrode in extending from the cathode to the anode. As an advantageous result, the electron transmission efficiency is not limited by the physical disposition of electrode structure in the electron flow path. Since substantially no current is therefore drawn by the control electrode, same may be driven by a relatively high, impedance control signal source. Further, the bulging electrostatic field productive of the electron flow path enables a relatively small potential swing in control electrode signal to be employed to control the flow of electron current between maximum and cut-off. By virtue of these advantageous characteristics, theirnproved electrode structure is particularly well suited to use in a high intensity pulsed X- ray tube, large amplification factor high power amplifier tube for switching applications, voltage regulation, and the like.

In the accomplishment of the foregoing and other objects and advantages, the electrode structure of the present invention generally includes a ringcathode coaxially spaced from an anode, a hollow cylindrical control electrode coaxially disposed inwardly of the cathode and having an outwardly flared annular rim extending radially about the proximal end portion of the cathode relative to the anode in coaxially spaced interposition to the cathode and anode, and ground shield electrode means including a hollow cylindrical shield electrode having an outwardly flared annular rim coaxially interposed between the cathode and control electrode to mask line-of-sight therebetween, and a hollow cylindrical ground plane can or baffle electrode disposed coaxially outward of the cathode and having a centrally apertured end wall in close longitudinally spaced relation to the rim of the control electrode so as to define a confined region of communication with the anode. When the anode is biased positive with respect to the cathode and an intermediate potential is applied to the control electrode, an electrostatic electron accelerating field is established which bulges coaxially outward about the rims of the shield and control electrodes in extending from the cathode to the anode. The baffle electrode confines the field to a restricted region in close proximity to the control electrode. The electron flow path follows the field and is thus not physically obstructed by electrode structure. When a negative potential is applied to the control electrode, the accelerating field is opposed in the bulging region thereof and electrons are repelled to the cathode. By virtue of the baffle electrode confining the field and electron flow path between the cathode and anode to a restricted region closely adjacent the control electrode, a relatively small swing in control electrode potential is required in preventing or enabling turning of the corner of the field bulge by the electrons for acceleration to the anode. I

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a semi-schematic isometric view with portions broken-away depicting the general configuration of electrode structure in accordance with the present invention.

FIG. 2 is a schematic representation of the electrode structure and associated electrical biasing means depicting the electrostatic electron accelerating field thereby established.

FIG. 3 is a sectional view taken at a diametric plane through a high power large amplification factor amplifier tube incorporating electrode structure in accordance with the present invention.

FIG. 4 is a sectional view taken at a diametric plane through a high intensity pulsed X-ray tube incorporating electrode structure in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings, FIG. 1 in particular, electrode structure 11, in accordance with the present invention, will be seen to include a ring cathode 12, of preferably circular transverse cross-section, and a cylindrical anode 13 coaxially inwardly spaced with respect to the cathode. The anode may be disposed in close longitudinal proximity to the ring cathode or longitudinally spaced therefrom, depending on the particular electron tube environment and application in which the electrode structure is to be employed. The electrode structure 11 further includes concentrically spaced hollow cylindrical control and ground shield electrodes 14 and 16 disposed coaxially outward of the anode and concentrically inward of the ring cathode. In accordance with the particularly salient aspects of the invention, the proximal ends of the control and shield electrodes 14 and 16 with respect to anode 13 terminate in outwardly flared annular rims l7 and 18 having conforming curvatures which partially surround the proximal end portion of the cathode with respect to the anode and are thus interposed therebetween. In other words, the control electrode 14 is coaxially interposed between the cathode l2 and anode 13, while the shield electrode 16 masks line-of-sight between the cathode and control electrode. In addition, the electrode structure includes a hollow cylindrical ground plane can or baffle electrode 19 coaxially disposed outwardly of cathode 12. The baffle electrode has a transverse end wall 20 with a central circular aperture 21 in close longitudinally spaced relation to rim 17 of control electrode 14. The baffle electrode thus confines communication between the cathode and anode to a restricted region in close proximity to the control electrode.

By virtue of the electrode configuration hereinbefore described, upon energization in the manner depicted in FIG. 2, the electron accelerating field and associated electron flow path between the cathode and anode are not physically intercepted by intervening electrode structure. More particularly, energization of the electrode structure is preferably accomplished by connection of the anode 13 to the positive terminal of a high voltage bias supply 22, the negative terminal of which is connected to cathode 12. The cathode and negative terminal may be both advantageously commonly connected to ground as shown. The shield electrode 16 and baffle electrode 19 are connected to ground, while control electrode 14 is connected to a bi-polar output terminal of a control signal source 23. The control source generates a step signal, as indicated at 24, which swings between negative and positive potential values. Typically, the anode is biased to a positive potential of the order of 30 KV with respect to the cathode, and the control signal swings between negative and positive potentials of the order of 300 and 500 volts, respectively. With the control signal positive, an electrostatic electron accelerating field is established between the cathode and anode substantially as indicated at 25. It is important to note that the field 25 coaxially bulges arcuately about the rim 18 of shield electrode 16 and rim 17 of control electrode 14 in extending from the cathode to the anode. In addition, the baffle electrode 19 restricts the field to a region in close proximity to the control electrode. Thus, the electron flow path, which corresponds to the accelerating field, is not physically intercepted by intervening electrode structure, as is the usual case with conventional electrode structure design. As a result, electrons are transmitted from the cathode to the anode with a relatively high transmission efficiency.

When the control signal 22 from source 21 is negative, the electron accelerating field is opposed and electrons are repelled to the cathode, thereby terminating transmission to the anode. It is of importance to note that by virtue of the curvature of the bulging field, and the close proximity of the control electrode thereto, a relatively small swing in the control signal potential between negative and positive values is required to prevent the electrons from turning, or to enable them to turn, the corner of the bulge for transmission to the anode. Since the required control signal swing is small relative to the anode bias potential, a relatively high amplification factor is provided by the electrode structure.

Because of the high electron transmission efficiency and high amplification factor attainable with electrode structure in accordance with the present invention, such structure is ideally suited to use in various electron tube devices, such as a high power, large amplification factor amplifier tube for switching applications, etc., as shown in FIG. 3, a high intensity pulsed X-ray tube as shown in FIG. 4, and the like.

Considering now the high power amplifier tube in detail with reference to FIG. 3, the tube will be seen to comprise electrode structure of the type hereinbefore described mounted within a sealed cylindrical vacuum envelope 26 of glass, or the like. More particularly, the electrode structure includes a ring cathode 12', a cylindrical anode 13', a control electrode 14' with outwardly flared annular rim 17', a shield electrode 16' with outwardly flared annular rim 18' and a ground plane or baffle electrode 19' mounted within envelope 26 in the previously described relationship. The control and shield electrodes are preferably provided as hollow cylindrical dish-shaped members respectively including circular base end walls 27 and 28 with cylindrical peripheral walls 29 and 31 projecting coaxially upward therefrom to terminate in the outwardly flared annular rims 17' and 18. The dish-shaped electrodes l4 and 16 are mounted in coaxially spaced nested relationship substantially centrally of the envelope interior. Such mounting is advantageously facilitated by means of a circular metallic mounting plate 32 coaxially secured within the hollow cylindrical bafile electrode 19' in spaced relation to the base end wall thereof. Plate securance may be accomplished by L-brackets 33 attached to the inner surface of the peripheral wall of the bafile electrode, and bolts 34 extending through the plate and brackets with nuts 36 threadably attached to their ends. The end wall 28 of shield electrode 16 rests upon plate 32 and is secured thereto, preferably by means of diametrically opposed posts 37 which also serve to support control electrode 14' in the previously noted coaxial nested relationship. More particularly, each post 37 is metallic and includes an intennediate shoulder 38 terminating in an externally threaded lowerend portion 39. The threaded portion 39 traverses an insulating feed-through bushing 41 which extends through aligned bores provided in the electrode wall 28 and plate 32. The threaded portion 39 also traverses an insulating washer 42 adjacent the under surface of the plate, and a nut 43 is threadably engaged therewith and tightened against the washer to thereby secure the post and shield electrode to the plate. The upper ends of the posts are tapped to receive screws 44 extending through apertures provided in the base end wall 27 of control electrode 14, which thereby serve to secure the electrode in position.

The ring cathode 12 is mounted coaxially outward of the peripheral wall 31 of shield electrode 16 in underlying spaced relation to rim 18' so as to be masked from the control electrode 14'. Mounting of the cathode in such position is preferably accomplished in a manner similar to that of the control electrode 14. In this regard, a plurality of metallic support posts 46 are mounted in upwardly projecting insulated relationship to plate 32 by means of insulating feed-through bushings 47, insulating washers 48, and nuts 49. Pins 51 depending from the ring cathode then engage bores formed in the ends of the posts and support the cathode in position.

With the cathode 12, control electrode 14' and shield electrode 16 thus mounted within baffle electrode 19' such that the apertured end wall 20 is in close coaxially longitudinally spaced relation to the rim 17' of the control electrode, the entire assembly of these electrodes is mounted within envelope 26 by securance of the lower end wall of the baffle electrode coaxially to the lower end wall of the envelope with, for example, a glass-to-metal joint. Mounting of the anode 13' in coaxially inwardly spaced relation to the control electrode 14' is then facilitated by securance of the anode to the upper end wall of envelope 26. The anode depends coaxially from the end wall of the envelope, through the bafile electrode aperture 21, to a position downwardly spaced from the plane of the control electrode rim 17. The anode advantageously extends exteriorly through the end wall of the envelope and is secured thereto as by means of a suitable sealed glass-to-metal joint.

Connection of the electrodes of the high power amplifier tube to suitable electrical energizing means may be accomplished by lead conductors (not shown) extending through the envelope 26 in sealed relation thereto in a manner well known in the art and through the lower portion of baffle electrode 19'. The conductors are readily interiorly connected to the lower ends of support posts 37 and 46 and bolts 34 to thereby provide connection to the control electrode 14, cathode 12, and shield and baffle electrodes 16' and 19', respectively. Electrical connection to anode I3 is made to the exteriorly extending end thereof.

When the cathode l2 and anode 13' are connected to an appropriate bias supply, the shield and baffle electrodes 16' and 19 are connected to ground, and the control electrode 14 is connected to a suitable control signal source, operation of the tube 24 proceeds in a manner similar to that hereinbefore described with reference to FIG. 2. Conduction through the tube occurs with high electron transmission efficiency and high amplification factor under the control of the control signal.

Referring now to FIG. 4, electrode structure in accordance with the present invention will be seen to be embodied in a high intensity pulsed X-ray tube 52. The electrode structure is in basic respects similar to, and mounted within a sealed cylindrical vacuum envelope 53 in substantially similar fashion as, the electrode structure of the previously described high power amplifier tube. The structure includes hollow cylindrical dish shaped control and shield electrodes 14" and 16", respectively defined by circular base end walls54 and 56 having cylindrical peripheral walls 57 and 58 coaxially projecting therefrom and terminating in outwardly flared annular rims 17" and 18". The electrodes 14" and 16" are mounted in coaxially spaced nested relationship upon a circular metallic mounting plate 59 by means of metallic posts 61. The end wall 54 of electrode 14" is secured to the posts by means of screws 62, and the posts traverse the end wall 56 of electrode 16" and plate 59 for securance thereto by means of nuts 63, insulating bushings 64 and washers 66 being interposed therebetween to insulate the posts from electrode 16" and plate 59.

A ring cathode 12" is disposed in coaxially spaced circumscribing relationship to the shield electrode 16" subjacent the rim 18" so as to be thereby masked from control electrode 14". Mounting of the cathode in such position is facilitated by means of metallic posts 67 secured to plate 59 with nuts 68 and insulated therefrom by means of insulating bushings 69 and washers 71. Support pins 72 depending from the cathode are secured to the posts 67.

The plate 59 with cathode 12", control electrode 14", and shield electrode 16 mounted thereon is coaxially secured, as by means of brackets 73 and bolts and nuts 74 and 76, within a hollow cylindrical metallic ground plane can or baffle electrode 19''. The electrode is coaxially mounted within the lower end of envelope 53 and in addition to the central circular aperture 21" at its upper end 20" is provided with a central circular aperture 79 at its lower end defined by a reentrant annular rim 81. The end walls 54 and 56 of electrodes 14" and 16" are likewise provided with central circular apertures 82 and 83 while plate 59 is provided with a central circular aperture 84. All of the foregoing apertures are in coaxial alignment with an X-ray transparent window 86 sealably mounted within a central circular aperture 87 formed in the lower end wall of envelope 53 at a position adjacent rim 81 of can 77.

The electrode structure of X-ray tube 52 also includes an anode 13" which extends through the upper wall of envelope 53 and depends therefrom into close proximity to the upper end wall 20" of electrode 19". The anode is coaxially aligned with X-ray window 86 through apertures 21", 82, 83, 84, and 79, and it is to be noted that the' anode is both inwardly and longitudinally coaxially spaced from the control electrode 14' on the opposite side of the baffle electrode end wall 20". The depending end of the anode is formed with a conical indentation 88 coated with a suitable X-ray target material, such as aluminum. In addition, the anode is preferably hollow to facilitate cooling by circulation of a coolant therethrough. It should also be noted that the anode 13 is advantageously of a material such as beryllium in order that its characteristic line may be readily removed from the X-ray spectra merely by employing beryllium as the material of window 86.

When the electrodes of X-ray tube 52 are energized in a manner subsequently described,'electrons from cathode 12" are accelerated coaxially outwardly around the rims of the control and shield electrodes 14" and 16", and through the aperture 21" of bafile electrode 19", to the target indentation 88 of anode 13". Due to the impact of the electrons, X-rays are emitted from the target through the window 86. In order to insure that the electrons impact the target indentation at the most favorable angle for X-ray emission, a field shaping electrode 89 is advantageously coaxially interposed between the control electrode 14" and anode 13''. More particularly, the

shaping electrode 89 is preferably of hollow cylindrical configuration and formed with an outwardly flared annular flange 91 at one end. The flange is seated in a spotface 92 formed in mounting plate 59 in circumscribing relation to aperture 84. The flange is lapped by the inner marginal portion of the shield electrode end wall 56 outwardly of its central aperture 83 and is thereby clamped in position. The shaping electrode extends coaxially through the shield and control electrode apertures 83 and 82 to the upper interior region of baffle electrode 19" adjacent aperture 21''. in the illustrated case, the second, or upper end of the shaping electrode is formed with an inwardly directed curved annular lip 93. The lip is appropriately contoured to shape the electrostatic electron accelerating field for the most favorable angle of incidence with the conical target indentation 88.

To facilitate electrical energization of the X-ray tube 52, lead conductors (not shown) may be extended through the wall of envelope 53 in sealed relation thereto and insulatedly through the wall of electrode 19" into its interior. One conductor may be internally connected on one of the mounting bolts 74 and externally connected to ground. The baffle electrode, shield electrode 16" and shaping electrode 89 are thereby placed at ground potential. A second conductor may be connected internally to one of the posts 61 and externally to a control signal source in order to apply a control signal to control electrode 14". A third conductor may be similarly connected internally to one of the posts 67 and externally to a grounded terminal of a high voltage bias supply. The positive terminal of the bias supply is then connected to the exteriorly extending end of anode 13 to thereby place the anode at high positive potential with respect to cathode 12'.

In the operation of the X-ray tube 52, a negative control signal on control electrode 14" repels electrons emitted from cathode 12" and thereby prevents their experiencing the force of the accelerating field to the anode 13". When the control signal on electrode 14" is pulsed positive, electrons from the cathode are accelerated to the target indentation 88 of the anode 13' along paths which bulge coaxially outwardly around the rims of the control and shield electrodes 14" and 16" in the manner previously described and thereby avoid the intervening electrode structure. As a result, the electrons impact the target with high transmission efficiency and thereby effect the emission of an X-ray pulse of correspondingly high intensity which is beamed coaxially through the interior of the shaping electrode 89 and exteriorly through window 86.

Although the invention has been hereinbefore described and illustrated in the accompanying drawings with respect to several preferred embodiments, it will be appreciated that various modifications and changes may be made therein without departing from the true spirit and scope of the invention, and thus it is not intended to limit the invention except by the terms of the appended claims.

What we claim is:

1. An X-ray tube comprising a vacuum envelope, a ring cathode disposed within said envelope, a cylindrical anode disposed within said envelope in coaxially longitudinally and inwardly spaced relation to said cathode, the proximal end of said anode with respect to said cathode having an X-ray target for emission of X-rays upon impact by electrons, a hollow cylindrical control electrode disposed coaxially disposed inwardly of said cathode and having an outwardly flared annular rim extending radially about the proximal end portion of said cathode relative to said anode in coaxially spaced interposition to said cathode and anode, a hollow cylindrical shield electrode having an outwardly flared annular rim coaxially interposed in spaced relation between said cathode and control electrode to mask line-of-sight therebetween, a hollow cylindrical baffle electrode disposed coaxially outward of the cathode and having a centrally apertured end wall coaxially interposed between said target and said. control electrode, means defining an X-ray transparentwindow in the wall of said envelope in coaxial alignment with said target, and means defining a line-of-sight path between said target and said window coaxially through said control, shield, and baffle electrodes, whereby biasing of said anode positive with respect to said cathode, grounding of said shield and baffle electrodes, and pulsing of said control electrode to an intermediate positive potential with respect to said cathode and anode effect coaxial flow of electrons from said cathode to said target for impact thereon and emission of X-rays therefrom through said window.

2. An X-ray tube according to claim 1, further defined by said target comprising a conical indentation in said proximal end of said anode having an X-ray target material coated on the surface thereof.

3. An X-ray tube according to claim 1, further defined by a hollow cylindrical field shaping electrode coaxially interposed between said control electrode and anode, said shaping electrode having a configuration to shape an electron accelerating field extending from said cathode to said anode for electron impact of said target at a favorable angle for X-ray emission.

4. An X-ray tube according to claim 3, further defined by said target comprising a conical indentation in said proximal end of said anode having an X-ray target material coated on the surface thereof, and said shaping electrode having an end adjacent said target formed with an inwardly directed annular lip.

5. An X-ray tube according to claim 1, further defined by said control and shield electrodes being dish shaped and respectively defined by circular end walls with cylindrical peripheral walls projecting coaxially therefrom and terminating in said outwardly flared annular rims, said end walls of said control and shield electrodes respectively having central circular apertures, a circular mounting plate coaxially secured to the inner peripheral wall of said baffle electrode in electrical connection therewith, said plate having a central aperture in coaxial alignment with said aperture in the end wall of said baffle electrode, said window, and said target, means mounting said control and shield electrodes in coaxially spaced nested relationship on said plate with said shield electrode in electrical connection therewith and said control electrode insulated therefrom, said apertures of said control and shield electrodes being coaxially aligned with said plate aperture, means mounting said cathode on said plate in insulated relation thereto, and means mounting said anode to depend from an end wall of said envelope to a position wherein said target is coaxially spaced from said end wall of said baffle electrode.

6. An X-ray tube according to claim 5, further defined by a hollow cylindrical field shaping electrode coaxially mounted on said plate in electrical connection therewith and projecting upwardly in coaxially spaced relation through said apertures of said shield and control electrodes to a position coaxially spaced from said end wall of said bafile electrode, said shaping electrode having a proximal end with respect to said target with a configuration to shape an electron accelerating field extending from said cathode to said anode for impact of said target at a favorable angle for X-ray emission.

7. An X-ray tube according to claim 6, further defined by said target comprising a conical indentation in said proximal end of said anode having an X-ray target material coated on the surface thereof, and said proximal end of said shaping electrode formed with an inwardly directed annular lip.

8. An X-ray tube according to claim 7, further defined by said anode being of hollow configuration to afford circulation of coolant adjacent said indentation. 

1. An X-ray tube comprising a vacuum envelope, a ring cathode disposed within said envelope, a cylindrical anode disposed within said envelope in coaxially longitudinally and inwardly spaced relation to said cathode, the proximal end of said anode with respect to said cathode having an X-ray target for emission of X-rays upon impact by electrons, a hollow cylindrical control electrode disposed coaxially disposed inwardly of said cathode and having an outwardly flared annular rim extending radially about the proximal end portion of said cathode relative to said anode in coaxially spaced interposition to said cathode and anode, a hollow cylindrical shield electrode having an outwardly flared annular rim coaxially interposed in spaced relation between said cathode and control electrode to mask line-of-sight therebetween, a hollow cylindrical baffle electrode disposed coaxially outward of the cathode and having a centrally apertured end wall coaxially interposed between said target and said control electrode, means defining an X-ray transparent window in the wall of said envelope in coaxial alignment with said target, and means defining a line-of-sight path between said target and said window coaxially through said control, shield, and baffle electrodes, whereby biasing of said anode positive with respect to said cathode, grounding of said shield and baffle electrodes, and pulsing of said control electrode to an intermediate positive potential with respect to said cathode and anode effect coaxial flow of electrons from said cathode to said target for impact thereon and emission of X-rays therefrom through said window.
 2. An X-ray tube according to claim 1, further defined by said target comprising a conical indentation in said proximal end of said anode having an X-ray target material coated on the surface Thereof.
 3. An X-ray tube according to claim 1, further defined by a hollow cylindrical field shaping electrode coaxially interposed between said control electrode and anode, said shaping electrode having a configuration to shape an electron accelerating field extending from said cathode to said anode for electron impact of said target at a favorable angle for X-ray emission.
 4. An X-ray tube according to claim 3, further defined by said target comprising a conical indentation in said proximal end of said anode having an X-ray target material coated on the surface thereof, and said shaping electrode having an end adjacent said target formed with an inwardly directed annular lip.
 5. An X-ray tube according to claim 1, further defined by said control and shield electrodes being dish shaped and respectively defined by circular end walls with cylindrical peripheral walls projecting coaxially therefrom and terminating in said outwardly flared annular rims, said end walls of said control and shield electrodes respectively having central circular apertures, a circular mounting plate coaxially secured to the inner peripheral wall of said baffle electrode in electrical connection therewith, said plate having a central aperture in coaxial alignment with said aperture in the end wall of said baffle electrode, said window, and said target, means mounting said control and shield electrodes in coaxially spaced nested relationship on said plate with said shield electrode in electrical connection therewith and said control electrode insulated therefrom, said apertures of said control and shield electrodes being coaxially aligned with said plate aperture, means mounting said cathode on said plate in insulated relation thereto, and means mounting said anode to depend from an end wall of said envelope to a position wherein said target is coaxially spaced from said end wall of said baffle electrode.
 6. An X-ray tube according to claim 5, further defined by a hollow cylindrical field shaping electrode coaxially mounted on said plate in electrical connection therewith and projecting upwardly in coaxially spaced relation through said apertures of said shield and control electrodes to a position coaxially spaced from said end wall of said baffle electrode, said shaping electrode having a proximal end with respect to said target with a configuration to shape an electron accelerating field extending from said cathode to said anode for impact of said target at a favorable angle for X-ray emission.
 7. An X-ray tube according to claim 6, further defined by said target comprising a conical indentation in said proximal end of said anode having an X-ray target material coated on the surface thereof, and said proximal end of said shaping electrode formed with an inwardly directed annular lip.
 8. An X-ray tube according to claim 7, further defined by said anode being of hollow configuration to afford circulation of coolant adjacent said indentation. 